You can use the terraform -target flag. Or You can have multiple terraform modules in a separate directory. And then you can terraform apply there. As an example, assume you have 3 .tf files separately. But you need to run more than just one of them at the same time. If you also, need to run them more often it's better to have a terraform module.
terraform
|--frontend
| └──main.tf
|--backend-1
| └──main.tf
|--backend-2
| └──main.tf
|--modules-1
| └──module.tf
Inside the module.tf you can define which files you need to apply.
module "frontend" {
source = "terraform/frontend"
}
module "backend-1" {
source = "terraform/backend-1"
}
Then issue terraform apply staying at the module directory.
And it will automatically import instances inside those paths and
apply it.